I have a question about getting a phone contract with Verizon Wireless?

I have one of those Casio G'zOne Boulder phones that someone gave me, but I don't have a contract with Verizon. Say if I were to get a contract with Verizon, would they make me buy a new phone even though I already have one I can use? Or would I be able to use my own phone from the start without having to buy a phone with the contract?